Cardiac CT halpern EDocFind com

• Editing of ECG Gating• ECG Gated Dose Modulation • Image reconstruction Cardiac Imaging Technique... Scan Start Position■ Native coronary arteries – Begin above carina – Tortuous aorta

Cardiac CT and CT Angiography: Techniques &

Clinical Applications

Ethan J Halpern, MD

Director, Cardiac CTThomas Jefferson University

• Editing of ECG Gating

• ECG Gated Dose Modulation

• Image reconstruction

Cardiac Imaging Technique

Patient Preparation

Prior to CT

■ Ask patient to refrain from stimulants (i.e coffee) on the day of the scan

■ No solid food for 4 hours prior to the study

■ Premedicate for asthma & allergic history

– Medrol 32mg po 12hrs and 2 hrs prior to study

■ Patient should have good IV access (18G antecubital)

■ Adequate EKG tracing – good contact

Patient Preparation - Heart

Rate

■ IV Beta Blockade (preferred)

– 2.5 – 30 mg Metoprolol

» Titrate to heart rate of 55-60

» Monitor BP while giving metoprolol– If asthmatic, consult physician

» No more than 10mg metoprolol

» Consider calcium channel blockers

■ Diltiazem (bolus 0.25mg/kg)

■ Oral Beta Blocker

– 50 – 100 mg Metoprolol

– 1 hour prior to examination

– Who will monitor the patient ?

Objective of the Contrast

■ HU in aorta

Contrast Injection

■ Use high iodine density contrast ≥ 350 mgI/mL

– We use Optiray 350 (Mallinckrodt Inc.)

■ 16 detector system (25-30 second scan)

■ Contrast volume = scan duration * injection rate

– Want sufficient contrast to enhance PDA at end of scan

Scan Start Position

■ Native coronary arteries

– Begin above carina

– Tortuous aorta or prominent

upper left heart border –

begin scan 1-2cm higher

■ Bypass Grafts

– Veins: top of arch

– LIMA: above clavicles

Scan Ending Position

■ Need to image PDA

– Note overlap of heart &

diaphragm

– Observe contour of heart

– Extend scan ~2cm below

the caudal extent of the

heart

– Position of heart will change

with inspiratory effort

Center the Scan on the

Heart

■ Maximize spatial

resolution for coronaries

– CT resolution is greatest in

the center of scan field

– Set left-right position on

AP scout view

– Move table up-down to

center on aortic root and

Left ventricle

• Higher Patient dose: dose proportional to ~ kV 2.7

• Longer recovery time between scans (shorter life)

Tube Current: mA/mAs

Axial: mAs = mA x Rotation-time/slice

Helix: mAs = mA x (Rotation-time/360°)/ Pitch

For most scanners: tube provides 300-500mA

A higher mAs means:

• Less noise: noise proportional to 1/(mAs)0.5

• Higher Patient dose: dose proportional to mAs

• Larger X-ray tube damage/scan

• Longer recovery time between scans

Scan Parameters

■ kVp

– Generally set at 120kVp

– For heavy patients (>200lbs) use 140kVp

– For patients with calcified arteries and stents also use 140kVp

■ mAs

– Effective mAs = mA x (rotation time / pitch)

– Effective mAs in the range of 700-900

– Increase for heavy patients to minimize noise

■ Pitch

– Generally 0.2-0.3, but adjust for heart rate

EKG Gating

■ Coronary CTA requires EKG gating to

overcome cardiac motion

■ Heart is most quiescent in mid-diastole and

EKG Based Techniques

■ Fixed time offset

– Example: 500 ms after R peak

Heart rate variation during CTA

Diastole varies in length

70%

Timing of Intervals in Different Heart Rates

■ Systole remains stable

■ Changes in heart rate primarily effect diastole

58 bpm r-r interval = 1021 msec r-t interval = 258 msec

104 bpm r-r interval = 576 msec r-t interval = 204 msec

79 bpm r-r interval = 757 msec r-t interval = 230 msec

Consistent Phase Selection

Beat-to-Beat Variable Delay

Algorithm

70%

■ Fixed time and percent

of R-R may not pick a consistent phase

■ Beat-to-Beat variable delay algorithm

– Always pick same

percentage delay in diastole

■ Improves image quality

58 bpm r-r interval = 1021 msec r-t interval = 258 msec

104 bpm r-r interval = 576 msec r-t interval = 204 msec

79 bpm r-r interval = 757 msec r-t interval = 230 msec

Single Heart beat Uses 180 o per heart beat

Temporal Res = (rot time)/2

Single Cycle

Reconstruction

Multi-Cycle Reconstruction

■ Combine a portion of

projections from one

heart cycle with a portion

of projections from

another to make the full

1800

■ Improves temporal

resolution, because each

segment of data covers

the same (smaller) region

in time

Single Cycle vs Multicycle

Dewey et al Investigative Radiol 39:223-229, 2004

Toshiba Aquilion 16-slice: 27/34 patients with HR>65

Temporal Window & Heart

Image Quality & Heart Rate

Hoffmann MHK: Radiology 234:86-97, 2005

Correction of Gating Errors

EKG Dose Modulation

■ Best images obtained at mid-diastole

– RCA sometimes is best at end-systole

■ Dose modulation can achieve dose reduction

Image Reconstruction

■ Reconstruction slice thickness

– 3mm for function

– 0.5-0.8mm for coronary arteries

– 1.0-1.2mm for photon limited scans

■ Reconstruction kernel

– Sharper kernel: noisier image, but may be

required to visualize coronary lumen with stents and calcified vessels

Slice thickness vs noise

Reconstruction filter vs noise

■ Choose appropriate filter

– Sharper filter for patients with heavy coronary calcium or stents

■ Perform targeted reconstructions

– 3mm reconstruction of contiguous slices @ 10 phases for cardiac function analysis

– 0.8mm reconstruction of overlapping slices @ 40%, 70%, 75% and 80% for coronary anatomy 1.0mm recons for heavy patients

Clinical Application of Coronary

Cardiac Indications

■ The MDCT angiography of the chest for cardiac assessment (0146T-0149T) is indicated for the following signs or symptoms of disease:

– Emergency evaluation of acute chest pain

– Cardiac evaluation of a patient with chest pain

syndrome (e.g anginal equivalent, angina), who is

not a candidate for cardiac catheterization

– Management of a symptomatic patient with known

coronary artery disease (e.g., post-stent, post CABG) when the results of the MDCT may guide the decision for repeat invasive intervention

– Assessment of suspected congenital anomalies of

coronary circulation

Bland-Altman Analysis of

Stenosis Grading

Dashed lines - 95% CI

Hoffmann: JAMA, Volume 293(20).May 25, 2005.2471–2478

Impact of Coronary

Calcium

Kuettner A et al Noninvasive detection of coronary lesions using 16-detector multislice

spiral computed tomography technology: initial clinical results JACC 44(6):1230-7, 2004.

All segments Ca Score < 1000

Proximal versus Distal

Segments All segments Proximal segs